Chore: add isValidThisArg in ast-utils.

[aladdin-add]

What is the purpose of this pull request? (put an "X" next to item)

[x] Other, please explain:

What changes did you make? (Give an overview)
isValidThisArg is used both in no-useless-call.js and prefer-spread.js. placed it in ast-utils in case some other rules will use it.

Is there anything you'd like reviewers to focus on?

[eslintbot]

LGTM

[eslintbot]

LGTM

[kaicataldo]

Thanks for the PR! This seems like a good idea to me, but I do think we need to add tests for any public methods we add to ast-utils. Can you add a suite of tests for isValidThisArg?

[not-an-aardvark]

IMO, it might be better to export equalTokens from ast-utils rather than isValidThisArg, because equalTokens seems like it's more useful in general. But I'm fine with either.

[aladdin-add]

IMO, it might be better to export equalTokens from ast-utils rather than isValidThisArg, because equalTokens seems like it's more useful in general.

maybe we can export both?

[not-an-aardvark]

I'm fine with that too, although I think isValidThisArg is fairly small and situational compared to equalTokens.

[kaicataldo]

I'm fine with both too as long as we have test coverage :)

[aladdin-add]

@not-an-aardvark @kaicataldo updated the PR to export equalTokens, and added tests.

[kaicataldo]

LGTM! Thanks for contributing.